It is proposed that studies undertaken over the past year, on the epidemiology of IUD retention, be continued. A considerable volume of knowledge has been accumulated with regard to use of IUD, but gaps in knowledge about IUD performance still exist, particularly as concerns reinsertion of IUD, the performance of later segments of IUD use, and the demographic impact of IUD. During the first year of studies, considerable progress was made with respect to knowledge about the reinsertion behavior of IUD. A compound distribution and a stochastic model have been developed to describe the performance of all segments of IUD. During a second year of studies, it is proposed that concentrated efforts be made to improve the stochastic model developed, using an alternating renewal process to describe the performance of the complicated process of "use and non-use" of IUD. Better methods to estimate the "potential fertility" of IUD acceptors will be explored, and methods of estimating the demographic impact of IUD will be developed.